This invention relates in general to parametric amplifiers and in particular to a photodiode amplifier suitable for use in detecting and heterodyning modulated light waves. Just as a radio wave carrier signal can be modulated to carry information, a light signal of frequency f can be utilized as a carrier that is modulated to carry information. Such modulated light signals can be utilized in laboratory experiments and can also be utilized to transmit data, for example, in optical fibers. In order to have high data rates, the optical carrier needs to be modulated at correspondingly high frequencies m. Often, these laboratory experiments give rise to frequencies m higher than can be amplified by conventional amplifiers so that it is necessary to heterodyne the modulation frequency down to an intermediate frequency (IF) that can be adequately amplified by conventional amplifiers.
In the most common present technique of detecting an heterodyning the modulation signal down to intermediate frequencies, the modulated light signal is detected by a photodiode to produce a photodiode signal proportional to the intensity of the light. For an amplitude modulated light signal, the photodiode signal is proportional to the modulation signal. This photodiode signal is sent to a separate mixer where the modulation signal is heterodyned to intermediate frequencies. The output of the mixer is connected to a conventional amplifier to amplify the output signal.
For modulation signals up to about 32 GigaHertz (GHz), the photodiode signal can be carried by coaxial cable. However, above 32 GigaHertz, it is necessary to utilize waveguides to carry the photodiode signal. Unfortunately, there is a rule of thumb that a given waveguide has a roughly flat transfer characteristic only over one octave (i.e., a 2:1 range of frequencies). Therefore, if the range of frequencies of interest is larger than one octave, then it is necessary to switch between waveguides utilized to carry the photodiode signal. Since the photodiode is typically built into the waveguide, this means replacing the photodiode as well as the waveguide. This makes it clumsy to perform experiments in which the modulation frequency is varied over more than one octave of frequencies above 32 GHz. The use of different waveguides and photodiodes in the various octaves also introduces some variations into the measurements. It would therefore by useful to have a scheme that enables more than one octave to be utilized above 32 GHz without necessitating interchanging waveguides and photodiodes.
In a technique presented by J. L. Hall and W. W. Morey in an article "Optical Heterodyne Measurement Of Neon Laser's Millimeter Wave Difference Frequency", Applied Physics Letters, Vol. 10, No. 5, Mar. 1, 1967, p. 152, the detection and heterodyning are both performed by a "hot carrier diode" that is photosensitive even though it was not designed to function as a photodiode. In that technique, the photosensitive diode is biased by a local oscillator (LO) signal so that the resulting photodiode signal has components at the intermediate frequencies that are equal to the difference between the frequencies of the modulation signal and the frequency of the LO signal. In that approach, the photodiode is strongly reverse biased into the multiplying region of the photodiode I:V curve where the reverse current becomes a strong function of voltage. This strong oscillating reverse bias varies the capacitance of the photosensitive diode so that it operates as a parametric amplifier. Unfortunately, this technique has a very poor efficiency--the incident light from a 1.5 MilliWatt laser produced only a 10.sup.-14 Watt output signal in which the signal to noise ratio is poor.